Degeneration of dopaminergic mesocortical neurons and activation of compensatory processes induced by a long-term paraquat administration in rats: implications for Parkinson's disease.

A deficiency of the dopaminergic transmission in the mesocortical system has been suggested to contribute to cognitive disturbances in Parkinson's disease. Therefore, the aim of the present study was to examine whether the long-term administration of a commonly used herbicide, paraquat, which has already been found to induce a slowly progressing degeneration of the nigrostriatal neurons, influences mesocortical dopaminergic neurons in rats. Paraquat at a dose of 10 mg/kg i.p. was injected either acutely or once a week for 4, 8, 12 and 24 weeks. Acute treatment with this pesticide increased the level of homovanillic acid (HVA) and HVA/dopamine ratio in the prefrontal cortex. After 8 weeks of administration paraquat increased the number of stereologically counted tyrosine hydroxylase-immunoreactive (TH-ir) neurons and their staining intensity in the ventral tegmental area (VTA), which is a source of the mesocortical dopaminergic projection. At the same time, few TH-ir neurons appeared in different regions of the cerebral cortex: in the frontal, cingulate, retrosplenial and parietal cortices. Chronic paraquat administration did not influence the level of dopamine in the prefrontal cortex but increased the levels of its metabolites: 3,4-dihydroxyphenylacetic acid (after 8-12 weeks), HVA (after 4 and 12 weeks) and HVA/dopamine ratio (4 weeks). After 24 weeks this pesticide reduced the number of TH-ir neurons in the VTA by 42% and of the Nissl-stained neurons by 26%, and induced shrinkage of this structure by ca. 25%. Moreover, TH-ir neurons in the cortex were no more visible after such a long period of administration and levels of dopamine metabolites returned to control values. The present results suggest that the long-term paraquat administration destroys dopaminergic neurons of the VTA. However, compensatory activation of the VTA neurons and cortex overcomes progressing degeneration and maintains cortical dopaminergic transmission.

A slowly developing dysfunction of dopaminergic nigrostriatal neurons induced by long-term paraquat administration in rats: an animal model of preclinical stages of Parkinson's disease?

The aim of the present study was to examine the influence of the long-term paraquat administration on the dopaminergic nigrostriatal system in rats. Paraquat was injected at a dose of 10 mg/kg i.p. for 4-24 weeks. We found that this pesticide reduced the number of tyrosine hydroxylase-immunoreactive neurons of the substantia nigra; after the 4-week treatment the reduction (17%, nonsignificant) was confined to the rostrocentral region of this structure but, after 24 weeks, had spread along its whole length and was approximately 37%. Moreover, it induced a biphasic effect on dopaminergic transmission. First, levels of dopamine, its metabolites and turnover were elevated (4-8 weeks) in the caudate-putamen, then all these parameters returned to control values (12 weeks) and dropped by 25-30% after 24 weeks. The binding of [3H]GBR 12,935 to dopamine transporter in the caudate-putamen was decreased after 4-8 weeks, then returned to control values after 12 weeks but was again decreased after 24 weeks. Twenty-four-week paraquat administration also decreased the level of tyrosine hydroxylase (Western blot) in the caudate-putamen. In addition, paraquat activated serotonin and noradrenaline transmission during the first 12 weeks of treatment but no decreases in levels of these neurotransmitters were observed after 24 weeks. The above results seem to suggest that long-term paraquat administration produces a slowly progressing degeneration of nigrostriatal neurons, leading to delayed deficits in dopaminergic transmission, which may resemble early, presymptomatic, stages of Parkinson's disease.

Neurotoxic action of 6-hydroxydopamine on the nigrostriatal dopaminergic pathway in rats sensitized with D-amphetamine.

To determine whether behavioral sensitization produced by prolonged D-amphetamine administration affects susceptibility of nigrostriatal dopaminergic neurons to the neurotoxic actions of 6-hydroxydopamine (6-OHDA), rats were treated daily from the 23 rd day after birth for 11 consecutive days with D-amphetamine (1.0 mg/kg s.c.) or saline. On the last day of treatment, one group primed with D-amphetamine and one control group of rats were tested to confirm behavioral sensitization development. The remaining animals were additionally treated on the 34 th day (one day after the last D-amphetamine injection) with 6-OHDA HBr (300 microg in 10 microl i.c.v., salt form, half in each lateral ventricle) or its vehicle. Four weeks later the levels of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-metoxytyramine (3-MT), as well as 5-hydroxytrypatmine (5-HT) and its metabolite 5-hydroxyindoleacteic acid (5-HIAA) were assayed in the striatum, by HPLC/ED. In rats with behavioral sensitization, 6-OHDA reduced endogenous dopamine and its metabolites content to a comparable degree in comparison to controls. This finding indicates that presumed up-regulation of the dopamine transporter in the behaviorially sensitized rats did not increase the neurotoxicity of a high dose of 6-OHDA.